1. FIELD OF THE INVENTION
The present invention relates to a multi-stage sprocket assembly for a bicycle.
2. DESCRIPTION OF THE RELATED ART
With a conventional multi-stage sprocket assembly, for shifting a drive chain from a smaller-diameter sprocket to a larger-diameter sprocket, the shifting movement of the drive chain occurs only at certain teeth of the larger and smaller sprockets having a predetermined positional relationship relative to each other. This means that the smaller sprocket has a certain number of teeth which satisfy this positional relationship. In the course of the above shifting action of the chain from the smaller sprocket to the larger sprocket, an inner plate portion of the chain remains at a tooth of the smaller sprocket until completion of the chain shift, i.e. until complete disengagement of the chain segment from the smaller sprocket. Whereas, the shifting chain first engages a tooth of the larger sprocket at its outer plate portion subsequent to the above-described inner plate portion. To facilitate understanding the essential spirits of the present invention, it is defined herein the above inner plate portion as a disengagement-interfering inner plate portion while the outer plate portion is defined as a engagement-leading outer plate portion. Similarly, the tooth of the smaller sprocket engaging with the trailing inner plate portion is defined herein as a disengagement-interfering tooth while the tooth of the larger sprocket engaging with the leading outer plate portion is defined as an engagement-leading tooth, respectively.
In the above-described chain shift from the smaller sprocket to the larger sprocket, the shifting chain segment is inclined relative to a drive rotation direction of the sprocket assembly. Then, for avoiding excessive interference between the shifting chain segment and the sprockets so as to allow the chain segment to move closer to the larger sprocket, the prior art has suggested provision of a convex portion adjacent a lower portion of the engagement-leading tooth of the larger sprocket or along an incoming path of the drive chain as a means for smoothing the chain shift (e.g. Japanese published utility model gazette No. 57-39270).
However, the prior art has paid no consideration to the configuration of the concave portion. As a result, the concave portion extends to an unnecessrily large area thus deteriorating the strength of the sprocket. Further, since the concave portion is provided at such position where the chain and the large sprocket overlap with each other with the chain substantially engaging both the smaller and larger sprockets, the chain shift action proceeds with the chain segment contacting a side face of the large sprocket until the leading end of the segment fits into the concave portion at which stepped edge the segment makes a sudden drop, which gives unnecesary vibration to the chain.
Accordingly, the primary object of the present invention is to provide a multi-stage sprocket assembly for a bicycle with an improvement which can minimize the amount of the concave portion thus achieving smooth chain shift action without impairing the strength of the sprockets.